Heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons. Heavy hydrocarbons possess very high viscosities and low API (American Petroleum Institute) gravities which makes them difficult, if not impossible, to pump in their native state. Additionally heavy hydrocarbons are characterised by high levels of unwanted compounds such as asphaltenes, trace metals and sulphur that need to be processed appropriately during recovery and/or refining.
A number of methods have been developed to extract and process heavy hydrocarbon mixtures. The method that is used most often commercially today for heavy hydrocarbon recovery from subterranean reservoirs is steam assisted gravity drainage (SAGD). In this method two horizontal wells are drilled approximately five meters apart then steam is injected into the reservoir through the upper wellbore permeating the oil sand. Steam softens the heavy hydrocarbon (e.g. bitumen) and enables it to flow out of the reservoir and into the lower well. From there it is pumped to the surface facilities. The transportability of the viscous heavy hydrocarbon mixture recovered is conventionally improved by dilution with a lighter hydrocarbon such as naphtha, a very light crude oil or a condensate (i.e. by addition of a diluent). The dilution of the heavy hydrocarbon with the diluent typically increases its overall API to about 20 degrees enabling it to be pumped to a refinery.
Nevertheless the SAGD process still suffers from inherent drawbacks. These include:    (i) the use of natural gas for steam generation causes high CO2 emissions whereas it has already been recognised in the energy industry that CO2 emissions must be managed better;    (ii) diluent is often added to transport the recovered hydrocarbon to refineries therefore large volumes of diluent must be transported and stored at extraction sites; and    (iii) higher levels of asphaltenes are present in the recovered hydrocarbon than non-heavy hydrocarbon and it has little commercial value.
There have been a number of attempts in the prior art to alleviate or minimise the above-mentioned disadvantages of conventional SAGD processing. U.S. Pat. No. 6,357,526 and WO2012/090178, for example, disclose processes and systems for producing heavy oil by SAGD wherein asphaltenes are separated from the crude heavy hydrocarbon and are ultimately used to generate steam.
Nevertheless a need still exists for steam-based recovery processes for hydrocarbon mixtures, and especially heavy hydrocarbon mixtures, which are less demanding in terms of steam generation and/or external energy required to recover and process the hydrocarbon. Methods that additionally reduce the need for external processing chemicals such as diluents would naturally be particularly beneficial.
The present inventors have now devised a steam-based method of recovering and processing a hydrocarbon mixture wherein at least some of the steam injected into the formation for hydrocarbon recovery is generated directly or indirectly from oxycombustion of a part of the recovered hydrocarbon mixture and another part of the recovered hydrocarbon mixture is used as a diluent in the processing of the recovered hydrocarbon mixture. The method of the present invention is therefore at least partially self-sufficient in terms of steam and also diluent.